Cylindrical drum-type wood chippers for reducing logs, branches, roots and the like to wood chips are well known. In general, drum type chippers comprise a rotating cylindrical drum having an exterior surface studded either with hammers or sharpened chipper knife blades depending on the desired consistency of the finished chips. Sharpened chipper blades, for example, tend to produce neatly cut wood chips while hammers tend to pulverize, shred, and tear the wood into randomly shaped shards. In use, logs and branches are fed to the rotating drum where the moving hammers or knives reduce them into small pieces that can be transported easily or be used for pulp, mulch, or the like.
Examples of these drum-type wood chippers are found in U.S. Pat. No. 4,802,631 of Arasmith, U.S. Pat. No. 4,785,860 of Arasmith, U.S. Pat. No. 1,418,735 of Plaisted, and U.S. Pat. No. 3,801,027 of Kubitz. In most of these examples, logs to be processed are fed to the surface of the rotating drum by a feeder mechanism such as a conveyor or feed roll. The blades of the drum are configured to impact, cut, and chip the log into pieces of roughly the same size, whereupon the pieces are discharged from the machine through a discharge chute.
In the Arasmith '631 example, a curved foramanous plate extends around the bottom of the rotating drum in spaced relationship therewith. As the wood chips are cut by the knives of the drum, they fall under the influence of gravity onto the foramanous plate. If their size is smaller than the holes in the plate, they fall through the plate and are discharged from the machine. However, if they are too large to pass through the plate, they may be picked up by successive knives as the knives move over the plate and carried back around the drum to be cut and reduced further in size. Alternatively, and preferably, these chips become lodged in the grate openings and are further cut and reduced in size as they are impacted by successive knives. Eventually, all of the chips are reduced to a size sufficiently small to pass through the plate. In this way, the maximum size chip produced by the machine is carefully controlled by the proper sizing of openings in the foramanous plate.
While drum chippers of the type just described have found widespread use in reducing wood to chips, they nevertheless have been plagued with numerous problems and shortcomings inherent in their respective designs. For instance, these types of chippers have traditionally been relatively large and heavy, which tends reduce their portability. Where they have been mounted on large trailers for transportation to a work site, the trailers have tended to be large and heavy, requiring a substantial and expensive towing vehicle. Further, these drum-type chippers have tended to be rather inefficient and have required large gas or diesel powered engines for their operation.
The inefficiency of drum chippers is heightened by the constraint that only the exterior surface of the rotating drum is usable since their interior surfaces are enclosed within the drum itself. Also, centrifugal force acting on the resulting chips in drum chippers naturally tends to propel the chips radially away from the drum surface. Thus, baffles and the like are required to constrain the chips to the vicinity of the chipper knives if any secondary cutting of the chips is to take place. Even with baffles, secondary cutting has not proven to be easily achievable with drum chippers. In fact, in some instances two drum chippers with different size knives have been used in succession to reduce wood to chips of a desired small size. Finally, the chipper blades on the surface of a drum chipper necessarily all move at the same velocity. However, it is well known that faster moving knives result in smaller chips than slower moving blades. Thus, reaping the advantages of different speed knives simply is not possible in the same operation with a drum-type chipper.
Disc chippers have been developed to address some of the problems associated with drum-type chippers. These disc chippers employ a rotating knife bearing disc rather than a cylindrical drum to reduce wood to chips. A good example of a disc chipper is presented in U.S. Pat. No. 4,827,989 of Strong. Other examples are illustrated in U.S. Pat. No. 1,195,774 of Brown, U.S. Pat. No. 3,732,907 of Nystrom, and U.S. Pat. No. 4,736,781 of Morey et al. In general, these devices comprise a housing that carries a rapidly spinning metal disc having knives mounted on one surface of the disc just behind gullets that pass through the disc. Logs to be processed are fed to the disc surface, usually at an angle, where the knives reduce the wood to chips. The chips, when cut, pass through the gullets in the disc. In this way, the chips move to the back side of the disc where they can be discharged from the machine. To the best of applicant's personal knowledge, all commercial prior art disc chippers include such knife and gullet arrangements.
While prior art disc chippers have been improvements over drum chippers for certain applications, they nevertheless have had their own set of problems and shortcomings. For instance, in some cases these chippers have not been able to provide accurate control of chip size since each chip is only cut once before it passes through a gullet in the disc and is discharged from the machine. This can be a particular problem when the cutters become dull. Also, as with drum chippers, there has been a natural lower limit to the size chip that could be produced in a single operation with prior art disc chippers and, in some cases, multiple operations have been required to reduce logs to chips of a desired small size. Finally, as can be seen from the above patent examples, complex knife designs have been invented to improve the efficiency or control the chip sizes in disc chippers. However, these high-tech knives are expensive to manufacture, require frequent sharpening or replacement, and can be utterly destroyed when encountering a stray rock or piece of metal that may be entangled or embedded in wood being processed.
Thus, there exists a continuing and heretofore unaddressed need for a disc-type wood hog that is efficient, economical, reliable, and that can reduce wood to chips or shards of virtually any desired small size in one single operation. Such a disc hog should do this without the need for expensive, high maintenance chipper knives and should provide for easy maintenance when necessary. It is to the provision of such a disc hog that the present invention is primarily directed.